Devices utilizing electrochromic materials can be put to use in a variety of applications. However, conventional electrochromic devices are susceptible to deterioration under conditions where ultraviolet rays are emitted. In order to prevent such a deterioration, it has been proposed to arrange an ultraviolet barrier on the outer surface of a substrate. However, such barriers are not satisfactory in view of aesthetic considerations, resistance to abrasion and durability.
It has been known for the purpose of imparting ultraviolet absorptivity to a transparent substrate, such as a glass plate, to use multiple reflections of a multilayer or use a transparent substrate coated with an ultraviolet absorptive material. The method of using the multiple reflections is superior to the latter because of free adjustability of wavelength to be cut off and the capability of clear-cut, but is disadvantageous in production cost due to the complicated processes of making the multilayer that is required.
There are two types of ultraviolet absorptive materials one of which is an inorganic ultraviolet absorber. Inorganic ultraviolet absorbers as disclosed in Japanese Patent Laid-Open Publication Nos. 5-339033, 5-345639 and 6-56466 are superior in resistance to weathering, heat-resistance and durability, but are disadvantageous in that these absorbers are less selective because the wavelength of the ultraviolet to be absorbed is determined by the band gap of a compound used as a constituent component of the absorber and it is not possible to absorb ultraviolets in a neighborhood of 400 nm. Furthermore, these absorbers are acompanied by unexpected coloration upon the interception of ultraviolet of longer wavelength.
On the contrary, organic ultraviolet absorbers can capture ultraviolet in various wavelengths by selecting a membrane through a type, a concentration and a thickness. As a result of extensive research directed to such organic ultraviolet absorbers, it has now been found that the use of absorbers which have a maximum absorption wavelength in a range of longer wavelength or which have an increased concentration or thickness in of membrane is conducive to intercept ultraviolets in a longer wavelength. However, such absorbers having the maximum absorption wavelength in an area of a longer wavelength as disclosed in Japanese Laid-Open Publication No. 6-145387 is poor in resistance to light and reduced in absorption power with the lapse of time. The absorbers disclosed in the above-mentioned Japanese Patent Publication No. 6-1415387 has a problem that the permeability is easily deteriorated due to the use of a fluorescent bleach.
Benzophenoic- or benzotriazolic ultraviolet absorber is improved in resistance to light and capable of absorbing ultraviolets of a relatively longer wavelength by increasing the concentration and the thickness of the membrane. However, in the case of coating a mixture of the absorber and a resin over a substrate, the membrane formed thereover is limited in thickness to an extent of several tens of micrometer. If an attempt is made to absorb ultraviolet rays up to longer wavelength with the absorber coated in this order of thickness, it is required to add the absorber in a fairy high concentration to the mixture. However, the mere addition of the absorber in a high concentration leads to problems involving the deposition thereof and bleedout due to the use over extended periods of time.
It has been attempted for solving these problems to react an absorber with a resin in which instance an absorber is copolymerized with an acrylic resin, as disclosed in Japanese Patent Laid-Open Publication Nos. 2-248412 and 6-88064. However, since an acrylic resin per se has a drawback in resistance to weathering and heat, the resulting ultraviolet absorber can not bear to be used over prolonged length of time.
Furthermore, a various researches have been made in the use of an ultraviolet absorber which are reactive with a silicone resin as disclosed in Japanese Patent Laid- Open Publication No. 61-54800, 2-117928 and 3-45094. It, however, has been found that most of such absorbers have a difficulty in synthesis in technical view and a problem in durability.
Furthermore, it has also been proposed in Japanese Patent Laid-Open Publication No. 62-148339 and 63-236016 to avoid a deterioration of an electrochromic smart window caused by ultraviolet rays with the use of an ultraviolet barrier disposed on the inner surface of a pair of transparent electrodes which have therebetween an electrochromic layer and an electrolyte layer and are disposed between a pair of transparent substrates facing each other, whereby the transparent substrates are affected in resistance to marring.
However, an ultraviolet barrier of a metal oxide disclosed in Japanese Patent Laid-Open Publication No. 62-148339 can not suppress a deterioration caused by ultraviolet rays because it fails to intercept near ultraviolet rays sufficiently. In the case of employing a dichroic layer as an ultraviolet barrier, there arises a drawback in production costs due to the requirement of multiple membrane formation processes.